In contrast to traditional wired networks in which computing devices are physically linked via copper or optical cables, wireless networks rely on radio frequency waves transmitted across an air medium.
Standards for controlling the behavior of such example wireless networks include the “Wi-Fi” family of standards currently defined in IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n and IEEE 802.11ac protocol.
In Wi-Fi networks, the network is created using a device known as an access point (AP). The access point transmits and receives radio frequency signals in specific frequency ranges called channels within the 2.4 GHz and 5 GHz spectrum or any other supported radio bands. For example, in 802.11g and 2.4 GHz 802.11n there are thirteen 20 MHz channels with a 5 MHz separation, with channel 1 centered at 2412 MHz and channel 13 centered at 2472 MHz. Devices such as smartphones, personal computers and tablet device which are compatible with the protocol used by the access point can join the network via an authentication and association process.
The access point is often paired to a router or modem hardware which is linked to a wide area network such as the Internet. It therefore functions as a bridge between the created wireless network and a wired network such as the Internet or a corporate network.
The benefit of a wireless network is that client devices do not have to be attached to the access point by a wire to gain connectivity to a wider area network, furthermore the devices can move while still maintaining connectivity. Due to the nature of the radio frequency signals and antenna properties, the wireless network created by an access point extends into the surrounding area, typically between 50 m indoors and 100 m outdoors under ideal conditions. Although the signal strength deteriorates as the distance from the access point increases and/or as the number of obstructions increases, so as to reduce these ranges in most cases, this degree of mobility provides flexibility to the client devices.
Due to the limited range of each access point and the availability of multiple frequency channels, it is common for many wireless networks to be present in any given area. By selecting different channels, the interference can be minimized. In some cases, each network is a private network and therefore each one will be secured with a pre-shared key security protocol such as Wi-Fi Protected Access version 2 (WPA2). It is also known to create hotspot networks such as the BT Wi-Fi network in which many access points are located across a large area and configured to appear as a single large area wireless network. In this case the access points within the hotspot network do not use any low level authentication since authorization onto the network is handled in the higher network layers using a protocol such as IEEE 802.1x or a web portal login.
Given that a wireless client device will often be within the connectivity range of multiple wireless networks, in order to determine which wireless networks are available, in accordance with the IEEE 802.11 family standards, a wireless device can perform a discovery process by sending a probe request frame on each available channel and wait for responses from any surrounding access points. This is shown in FIG. 1. Conventionally and as shown in FIG. 2, the access points are configured to respond to any probe messages with capability information and supported data rates. The client device will then process any received responses and present the list of networks to a user for manual selection. Once a network has been selected, then the standard authentication and association procedures are carried out until connectivity to the access point is established. Furthermore, the client device may remember access points it has previously connected to and when a known or “preferred” network is detected, it will automatically initiate the authentication and associated process without any action by the user.
Where there is a large number of access points, the number of probe request frames and probe responses can be very large. This reduces the bandwidth available for actual data traffic since only one device can be transmitting on a given channel at one time in accordance with the collision avoidance defined in Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) protocols. This is especially true in densely populated areas, or at large events such as sporting events.